A type of visual display device coming into prominent use incorporates so-called liquid crystals. Liquid crystals are a class of organic liquids which exhibit some properties of ordered crystalline structure within certain temperature ranges. One class of liquid crystals now receiving considerable attention is nematic liquid crystals which when used in a dynamic scattering mode are transparent to visible light but will become cloudy or opaque as a result of electrical current flow therein. The current flow is believed to rearrange the orientation of the liquid crystal molecules resulting in scattering of incident light. Field effect operation of liquid crystals is also known, such as the so-called "twisted nematic" mode. Some of the materials suitable for use in Liquid Crystal Displays (LCD's) are disclosed in U.S. Pat. No. 3,716,289 by L. T. Creagh et al, and U.S. Pat. No. 3,655,270 by L. T. Creagh, both assigned to the assignee of the present application.
Visual displays of letters or numbers can be formed using a thin (0.5 to 1.0 mil) layer of a dynamic scattering mode nematic liquid crystal sealed between two sheets of glass. For the reflective type of display panel conductive transparent electrodes on the inside surface of the front sheet and a transparent or highly reflective electrode on the inner surface of the back sheet are provided for each digit. When a transparent electrode is used on the back sheet, a highly reflective medium, such as aluminum foil, for instance, is preferably disposed on the rear of the sealed display unit. When a voltage is applied to the opposing electrodes causing a current to flow in the liquid between the electrodes, the liquid loses its transparency in that region thus presenting to an observer an image in the shape of the electrodes from transmitted or reflected light. For the transmissive type of display panel the structure is similar, however, the back electrodes must be transparent and a light source is placed behind the panel.
In twisted nematic field effect LCD's, a similar structure is used, but light polarizing media, analyzer and polarizer, are used with the front and back sheets of glass, respectively. Linearly polarized light propagating perpendicular to the display is rotated 90.degree. as it passes through the liquid crystal when no external electric field is being applied. By crossing the analyzer and the polarizer, light will pass into the display, reflect off the reflective medium and pass back out of the display when no field is present. By energizing appropriate segments, dark characters on a light background appear. By arranging the analyzer and polarizer in a parallel configuration, light is permitted to pass through the display only in presence of an electric field, then by energizing appropriate segments, light characters on a dark background appear.
The voltage applied may be either D.C. or A.C., but A.C. voltages are preferably used with liquid crystal displays because it has been found that D.C. voltages tend to shorten the useful life of LCD's compared to A.C. voltages.
Further, the pairs of opposing electrodes may be arranged such that all of the segments of the display are energizable at the same instant, as suggested by the electrode arrangement disclosed in FIG. 1 of U.S. Pat. No. 3,771,855, which patent is assigned to the assignee of this invention. Still further, displays using multiplexed scanners for driving LCD's are known in the prior art, for example, U.S. Pat. No. 3,999,180 teaches a multiplexed LCD system. But these systems when used with eight character displays have not been compatible with chips disposed in standard twenty-eight pin packages. For example, in calculator applications, two of the twenty-eight pins are set aside for the power supply (i.e. battery) connections to the chip, leaving twenty-six pins for driving the display. However, when the eight character, eight segment (seven segments for the digit and one segment for the decimal point) per character position LCD is utilized, the prior art multiplexing methods for driving the display have required more than twenty-six pins.
It was, therefore, one object of this invention to improve the multiplexing methods used with LCD's. It was another object to dispose a calculator chip capable of driving an eight character, eight segment per character, LCD in a standard twenty-eight pin package.
The foregoing objects are achieved as is now described. For an LCD responsive to a one-third duty cycle drive, a total of sixty six segments are provided with twenty two segment select lines and three drive lines. The sixty six segments are arranged into eight characters of seven segments per character (56 total segments), and either eight segments for decimal points for each character position, a minus sign segment and an annotator segment (e.g. memory indicator or error indicator) or seven segments for decimal points for all but the least significant digit position, a minus sign segment and two annotator segments. The total number of pins for such light character position LCD's is 25.
Other embodiments of liquid crystal displays are taught as well as a method for actuating the display and strobing a keyboard corrected to the segment drive lines.